1. Field of the Invention
The present invention relates to a gas sensor for measuring gas components such as NO, NO2, SO2, CO2, and H2O, contained in the atmospheric air or the exhaust gas discharged from vehicles or automobiles, for example. In particular, the present invention relates to a gas sensor which has a protective cover surrounding a sensor element.
2. Description of the Related Art
A variety of gas sensors have been suggested and practically used. For example, there are oxygen sensors based on the use of oxygen ion conductors, NOx sensors (see Japanese Laid-Open Patent Publication No. 8-271476), HC sensors (see Japanese Laid-Open Patent Publication No. 8-247995), hydrogen sensors based on the use of proton ion conductors, H2O sensors, oxygen sensors based on the use of oxide semiconductors such as SnO2 and TiO2.
Among the gas sensors as described above, the oxygen sensor based on the use of ZrO2 and the oxygen sensor based on the use of TiO2 are widely used for controlling the oxygen concentration in the exhaust gas discharged from an automobile and/or controlling A/F (air-fuel ratio), because such gas sensors operate stably even in the environment of the exhaust gas discharged from an automobile. The NOx sensor, which is based on the use of ZrO2, is also at the stage of practical use to control NOx discharged from an automobile.
As an oxygen sensor attached to an exhaust tube of an internal combustion engine, a sensor having a protective cover provided around a sensor element to bring about a uniform flow of the exhaust gas, or a sensor having a protective cover for avoiding condensed water produced upon the start-up of the engine (droplets of water) is used. Sensors described in U.S. Pat. Nos. 4,597,850 and 4,683,049 are known as oxygen sensors to each of which a protective cover of a double structure is attached.
As for the conventional protective covers as described above, if a water protective cover is used, the response of the gas sensor may be delayed. Accordingly, it has been suggested that an inner protective cover of a double-structure protective cover, which is disposed close to a sensor element, has inner gas inlet holes facing the sensor element for improving the response performance (Japanese Patent No. 2641346).
However, in the conventional protective cover, the gas sensor is supposed to be positioned upstream with respect to the catalyst. If the sensor and the cover are installed downstream with respect to the catalyst, a problem was found in relation to water resistance (performance to avoid condensed water produced upon the start-up of the engine). Further, another problem was found in relation to water resistance depending on the angle of attachment to the exhaust tube of the internal combustion engine.
In order to obtain the quick response performance, it is conceived to provide a structure in which the inflow amount of the measurement gas is increased. However, in this structure, the droplets of the condensed water produced upon the start-up of the engine also tend to come into the sensor element. That is, it is difficult to balance preventing the sensor element from the droplets of water with improving the response performance.
Further, considerable temperature change or fluctuation may occur in the sensor element as the measurement gas flows into the protective cover, and cracks may appear in the substrate of the sensor element.